Radio communication systems typically use a receiver (e.g., a selective call receiver or "pager") that has at least one unique call address associated therewith. These radios receive and correlate an address to the at least one unique call address, then decode an optional message and alert a user. After being addressed, the receiver preferably decodes and presents at least a portion of a received message via a presentation means. Considering the preceding discussion, one of ordinary skill in the art can appreciate that radio communication systems are an excellent vehicle for delivering voice, numeric, alphanumeric or coded information.
Contemporary radio receivers sometimes employ an automatic gain control (AGC) to control distortion resulting from a signal overload condition. Typically, an AGC receiver adjusts a gain of a radio frequency amplifier in a receiver section to reduce signal distortion. A variety of AGC detection schemes can be used to control the adjustment process. Two examples of prior art AGC detection schemes are monitoring a received signal strength indicator (RSSI) and monitoring a current drain in the radio frequency amplifier. In the case of a RSSI, a relative signal strength is detected and compared to a predetermined level. If the detected signal strength exceeds the predetermined level, the AGC is activated, thereby reducing the gain of the radio frequency amplifier and in most cases, eliminating the signal overload condition. The current drain monitoring topology functions in much the same manner, controlling the gain of the radio frequency amplifier in response to a relative (or absolute) current drain detected. A detailed example of a prior art intermodulation controlled communication system employing a form of AGC is discussed in U.S. Pat. No. 5,001,776 entitled: "COMMUNICATION SYSTEM WITH ADAPTIVE TRANSCEIVERS TO CONTROL INTERMODULATION DISTORTION", issued to Edward T. Clark and assigned to Motorola, Inc.
One of ordinary skill in the art will appreciate that the preceding methods and apparatus must operate dynamically to effectively implement their respective AGC schemes. Dynamic operation allows these apparatus respond to signal strength changes in "real time." However, because of the sensing methods used, each of the preceding approaches operate at fairly constant high power levels, thus causing a considerable decrease in battery life if implemented in a portable paging receiver. Furthermore, because of the size constraints imposed on state-of-the-art portable paging receivers, the only alternatives available for powering these receivers are a single cell AA or AAA battery. Since the RSSI, local oscillator, or radio frequency amplifier current sensing topologies discussed operate on five or more volts D.C., their implementation in a portable paging receiver is undesirable. It is clear that an alternative topology and method must be found to allow a portable paging receiver to implement a signal quality control mechanism, thereby improving its operating characteristics and usefulness in a high interfering signal strength environment.